This invention relates to rolling bearings for use in automotive vehicles, agricultural machinery, construction equipment, machinery for use in the iron and steel industry, etc., and more particularly to rolling bearings of this kind which are required to have long service lives.
Conventionally, there have been used various alloy steels, such as Bearing steel Class No. 2 (SUJ 2), for this kind of rolling bearing. However, recently, rolling bearings of this kind are often used under conditions in which higher pressure is applied to the bearings, as heavier loads are applied thereto and/or the size of same is reduced with enhancement in the performance of machinery using the rolling bearings. Accordingly, there is an increasing demand for rolling bearings having higher durability and hence prolonged lives.
In an attempt to prolong the lives of rolling bearings of this kind, it has been employed to optimize the concentration of retained austenite in the raceway surface or the rolling contact surface of the rolling element, and reduce the non-metallic inclusion. These measures have somewhat contributed to prolonging the lives of the rolling bearings. However, they are not sufficient to fully meet the above-mentioned recent demand for bearings having prolonged lives.
To enhance the hardness of component parts, which are formed of steel, of a rolling bearing is one possible way to improve the fatigue resistance of the rolling bearing for the purpose of prolonging the life of same. However, to enhance the hardness of the steel, carbide-forming elements, such as Mo, W, and V, have to be added in large quantities, as is the case with high speed tool steels. This results in crystallization of large carbides upon solidification, from which fatigue is liable to start to occur. Therefore, this method cannot necessarily attain the desired prolongation of the lives of the rolling bearings.
Under these circumstances, methods have recently been proposed e.g. by Japanese Provisional Patent Publications (Kokai) Nos. 61-19756 and 60-67644, which comprise preparing a fine powder of component elements for a high speed steel by the atomizing method, and then sintering and rolling same, without using the conventional casting method, as in the case of sintered high speed steel, to thereby obtain a very hard steel containing no large carbides. According to the method proposed by Japanese Provisional Patent Publication (Kokai) No. 61-19756, a powder of a carbide of a carbide-forming element selected from the group consisting of Cr, Mo, W, and V is mixed with a powder of soft iron (Fe) and carbon, and the resulting mixed powder is sintered to form a sintered high speed steel. According to the method proposed by Japanese Provisional Patent Publication (Kokai) No. 60-67644, a powder of an alloy is evenly mixed with particles (non-reactive particles) which do not react with the matrix phase metal during sintering and heat treatment, then the resulting mixed powder is sintered, and then the resulting sintered mixture is sintered together with particles (reactive particles) which precipitate during heat treatment to form a sintered high speed steel.
Sintered high speed steels produced by the proposed methods contain very small crystallized carbides, by precipitation and solidification of steel without forming large carbides, and hence are very hard and hence excellent in wear resistance. However, these methods do not take into consideration the upper limits of the size of pores formed in the steel and the size of carbides, the optimum range of hardness, etc. The mere use of the very hard sintered high speed steels to form rolling bearings without taking these factors into consideration cannot attain desired prolongation of the lives of the rolling bearings.